Allow dynamic changes to the window, e.g. performed by the user, and
make sure to refresh its contents, while preserving aspect ratio.

In practice, Expose and ConfigureNotify events are tracked in X11
display mode by default. This occurs in a separte event thread, and
this is similar to what xvimagesink does. Any of those events will
trigger a reconfiguration of the window "soft" size, subsequently
the render-rect when necessary, and finally _expose() the result.

The default of handle_events=true can be changed programatically via
gst_x_overlay_handle_events().